Arachidonic acid cyclooxygenase metabolites, thromboxane B₂ (TXB₂), prostaglandin E₂ (PGE₂) and 6-keto-prostaglandin F₁ (6-keto-PGF₁) were measured in horses where anaesthesia was maintained with halothane. Two horses suffering from postanaesthetic myositis were compared with four normal horses. TXB₂ and PGE₂ levels were higher in mixed venous blood drawn from the myopathic horses. An increase of TXB₂ and PGE₂ levels appeared when myopathic horses were rolled into dorsal recumbency after a prolonged period of lateral recumbency. One hour after the end of anaesthesia, TXB₂ had continued to increase whereas PGE₂ decreased. By measurements on blood samples drawn from the brachial vein, we have shown that the rising level of TXB₂ in mixed venous blood is mainly due to the increase of TXB₂ in blood draining the dependent leg. The origin of the rise in PGE₂ is not demonstrated in this study. 6-keto-PGF₁ did not change during anaesthesia. An explanation of this imbalance between TXB₂ and 6-keto-PGF₁ production is considered.